PTFE porous tubes made by a stretching method have been used as tubular prostheses, and many cases of the clinical application of such tubes as artificial blood vessels have been reported. Such PTFE porous tubes are superior over conventional prostheses made of knitted or woven fabrics. The stretched PTFE tube has a microfibrous structure consisting of small nodes interconnected with many thin fibrils. The diameter of the fibrils, which depends on the conditions of the stretching or drawing operations employed, can be made much smaller than the filaments in the knitted or woven fabrics. In addition, because their pore size and porosity can be freely controlled, the PTFE tubes are flexible and can be used as substitutes for tubular organs such as blood vessels with little chance of thrombus formation. In addition to these advantages, the PTFE tubes allow pseudo-intima to form on the surfaces of the internal cavities thereof without causing any damage in the surrounding tissues. For these reasons, the stretched PTFE tubes are considered to be one of the most promising prostheses for replacement of tubular organs.
Relatively large tubes having an inner diameter of 6 mm or more have been demonstrated to perform well as artificial blood vessels, but with narrower tubes having an inner diameter of 5 mm or less, growth of thrombi occuring on the inner surfaces thereof, particularly at the sites where the tubes are sutured with host blood vessels has been reported. Therefore, at the present time there are no PTFE tubes available to replace any blood vessel, particularly blood vessels having an inner diameter of less than 5 mm, more particularly as narrow as 1 to 3 mm even if the tube has the same fibril structure as the artificial blood vessels having an inner diameter of 6 mm or more.